Alright, based on what I see played, and what seems to pop up on the Pokemon website/PokeBeach/etc. Please don't get mad at me if something is wrong. I am not an expert, but I think I have a decent idea of the tiers:
Tier 1 ("Meta"):
Gardevoir GX
Garbodor/variants
Volcanion/variants
Metagross/some variants
Darkrai (Maybe)
Golisopod (Maybe)
Marshadow/variants (A case could be made for this to be in Tier 2)
Tier 2:
Ninetales
Tapu Bulu
Greninja
Tier 3:
Aqua box (No idea what this is, so imma stick it here for now)
Tapu Koko GX
DeciduTails
Sylveon
Probably not very playable/Really not in a tier:
Xerneas BREAK (I am not sure this is very playable anymore...)
Decidueye/Ninetales (This deck, as far as I am concerned, is not nearly as good as it was, mostly due to loss of FoGP. Rare Candy variants work, but are slow and clunky...)
Once again, this is more from my point of view. For some of these, a case could be made for some decks to be in other tiers.
Thanks,
-AV